Federal law enforcement officials are reporting a rise in attacks in which the telephone lines of emergency call centers are flooded with bogus calls by extortionists whose demands for cash are refused.

The Department of Homeland Security (DHS) and the Federal Bureau of Investigation (FBI) recently issued an alert on the bizarre scheme first reported by the blog KrebsonSecurity. Dozens of recent attacks have been reported against the call centers, which handle administrative and emergency calls for police, firefighting and ambulance services. Emergency 911 lines were not affected.

So-called telephony denial-of-service (TDoS) attacks have been on the rise against public and private organizations, according to a recent report by SecureLogix, which provides TDoS mitigation services. Typically, the motivation is to extort money or to protest a particular political or social cause.

"The entire number of attacks is rising," said Rod Wallace, vice president of services for SecureLogix. The increase is seen across organizations, public and private.

In the latest attacks, someone with a heavy accent calls the center, known as a public-safety answering point, claiming to be with a collections company for payday loans, the federal alert says. The caller then demands a payment of $5,000 to cover the outstanding debt of a former employee or sometimes for someone who never worked at the center.

"They may stop for several hours, then resume," the alert said. "Once attacked, the attacks can start randomly over weeks or months."

Emergency call centers are being targeted because of the importance of the operations, which are sometimes handled by private organizations with agreements that could carry penalties if a certain level of service is not maintained, Wallace said.

Last month, the Louisiana State Analytical and Fusion Exchange, a center for distributing information across law enforcement offices, reported a similar extortion scheme against two public sector entities, including a 911 call center.

"The perpetrators of the attack launch numerous phone calls against the target network, tying up the system from legitimate calls," according to the LA-SAFE advisory provided to CSO by SecureLogix.

Launching TDoS attacks are relatively easy because of the many software tools available to automate the dialing of bogus calls through Internet telephony applications.

Social networks such as Facebook and Twitter are increasingly being used to organize large numbers of callers to harass organizations seen as the bad guys in a political or social cause. Such activism occurred during the "Occupy" movements last year, SecureLogix said.